Our recent results have demonstrated that over 50 percent of total platelet protein consists of 3 contractile proteins: myosin, actin (form I and form II) and a high molecular weight Actin Binding Protein. It has also been reported that stored platelets contain a high percentage of partially degraded myosin. There are two related objectives of this project. First, to evaluate the extent of proteolysis of platelet contractile proteins by intracellular protease during platelet storage; to determine the effect of this proteolysis on platelet contractile function; and to test the effect of low molecular weight, non-toxic, specific inhibitors of intracellular proteases on proteolysis and maintenance of function. Second, to characterize the interactions of actins and Actin-Binding Protein in order to test our hypothesis that stimulation of platelets is accompanied by dissociation of the complex of actin and Actin-Binding Protein, resulting in formation of new microfilaments in pseudopods.